The permeability of ferromagnetic materials is influenced by mechanical stresses. This physical effect which is known as the magneto-elastic effect can be used to measure forces which act on an item and cause stresses in this item. For this purpose, the item is provided with a ferromagnetic layer if the item does not itself consist of a ferromagnetic material or does not contain a ferromagnetic layer. Magneto-elastic force sensors can then measure the stresses caused by an acting force using the magneto-elastic effect, the force causing the stresses being able to be determined from the measurement result. Magneto-elastic force sensors based on the magneto-elastic effect can be used, inter alia, to measure torques in a rotating object since a torque generates stresses in the rotating object.
A number of different sensors which use the magneto-elastic effect to contactlessly record torques of shafts, for example, are known. The magneto-elastic sensors are distinguished by a high level of accuracy, in which case recalibration is required just as little as the approaching of reference marks.
A magneto-elastic torque sensor which is used to measure the torque of drive shafts is known from DE 10 2009 008 074 A1, for example. Said document describes a measuring arrangement for recording the torque of a shaft, which arrangement comprises a torque sensor which is positioned at a predetermined gap distance from the surface of the shaft.
In order to measure torques on force-transmitting shafts of machines, an electromagnetic coil is contactlessly arranged at a short distance from the surface of the shaft and reacts to the change in the permeability in a ferromagnetic layer on the shaft or a shaft which is ferromagnetic per se with a signal change. For this purpose, the shaft must be exposed to torsional stresses, that is to say the magneto-elastic torque sensor must be arranged along the shaft between a driving torque and the reaction torque working in opposition to the latter. The shaft formed from the ferromagnetic material is thus part of the measuring apparatus.
If the position of the shaft is changed during measurement, the measurement result which reacts very sensitively to the distance between the sensor and the shaft is influenced by the position changes. Movements by a few tenths of a millimeter perpendicular to the axis of rotation of the shaft already considerably influence the measurement result of the torque sensor. Such position changes of the shaft cannot be eliminated with any economically reasonable technical outlay, in particular in technical applications in which large shafts are used for force transmission.
DE 10 2009 008 074 A1 has therefore proposed the practice of positioning the torque sensor on the shaft using a ring which engages around the shaft. The ring is also connected to a housing section in order to protect it from being concomitantly rotated with the shaft. In this case, the ring is fastened to the housing section in a manner pivotable about a pivot axis and displaceable in the radial direction of the shaft, with the result that it can participate in movements of the shaft. The ring is mounted on the shaft in this case by means of a bearing bushing, sliding shells or ball bearings.